tube amp popping

i was using my friends guitar amp head and cabinet (with gutiar), and it started making this weird popping sound. i turned down the channel volume and it still made the sound, and when i turned down the master the popping got softer. do any one of you helpful people know what could have made this popping sound?

Additional info: its an old acoustic guitar amp head, running on 4 ohms, with a 4 ohm cabinet, on 50 watt setting, i tried it with my 8 ohm cabinet on the 8 ohm setting and it made the same noise.

Having been a pro technician, there is a 99% chance it's a capacitor like JMX said. They heat up and start breaking down. That's why it happens only after it's been on for a while.

The fact you still hear it softer at low volume tells us that the bad cap is in some stage before the master volume control.

Only if you are technically inclined, you can find out which one it is by hitting each electrolytic or old paper capacitor in the early stages with freeze spray. The popping will lessen or change radically when you zapped the right one, then just replace it.

the other 1% chance is a tube has gone "gassy". That will cause "frying eggs" hiss and popping. But this is easy to check for. Turn out the room lights and look to see which preamp tube is glowing purple.